1. Field of the Invention
This invention relates to a process for economically producing tin (II) sulfate having a low chloride and tin (IV) content.
2. Description of the Prior Art
Tin (II) sulfate is employed in the galvanizing industry today for tinning a variety of objects. However, it is especially used for tinning household articles, printed circuits, electrical articles and consumer articles, such as, screws, can openers and the like.
Also, for reasons of toxicity and because of legislation dealing with foodstuffs, cadmium plating and zinc plating are being increasingly replaced by galvanic tinning.
Galvanic tinnings are frequently conducted with the so-called "soluble anodes" wherein metallic tin, frequently in the form of a rod or plate, is electrolytically dissolved in dilute sulfuric acid. At the same time, the object to be tinned or the objects to be tinned are inserted directly into the circuit as the cathode and, by means of such an arrangement, the tin is deposited directly onto these objects.
Such galvanic tinning baths must initially be brought to the required operating concentration by the addition of tin (II) sulfate either in the form of a solution or as the solid salt. This adjustment of concentration is necessary because tin is dissolved and deposited at practically the same rate and it is therefore not possible to build up significant concentrations of tin (II) sulfate in the respective solutions.
However, losses occasioned by tin (II) sulfate solution adhering to the tinned objects as they are removed from the tinning bath, must again be replaced or compensated for by the occasional addition of tin (II) sulfate.
Tin (II) sulfate has also been employed very successfully in the metallic-salt dyeing of aluminum for some years.
Because of the quality of the finishing agent and the simplicity and economy of the galvanic or electrolytic process, it is necessary, in these fields of application, to use tin (II) sulfate that has a low chloride and a low tin (IV) content and which, in addition, is obtained in as economical a manner as possible.
According to the state of the art, tin (II) sulfate is obtained by reacting tin (II) chloride which is obtained by dissolving tin in hydrochloric acid, optionally with an excess of metallic tin, while simultaneously passing in chlorine or by reacting tin with tin (IV) chloride, with soda solution to form tin (II) oxide (tin suboxide) and reacting tin (II) oxide with dilute sulfuric acid. The tin (II) sulfate so formed is separated from the mother liquor.
Also, the electrolytic production of tin (II) sulfate by anodically dissolving tin in aqueous sulfuric acid is well known.
In so doing, processes are known which operate without the use of membranes and in which electrode polarity reversal is effected at certain time intervals as well as processes which operate with organic membranes or organic anion-exchange membranes.
An electrolytic process for producing tin (II) sulfate without the use of membranes is described in the publication, J. Electrochem. Soc. Electrochemical Technology, Feb. 1971, Vol. 118 and processes using organic membranes or organic anion-exchange membranes are described in the Russian Pat. Nos. 141,860 and 157,342 as well as in the U.S. Pat. No. 3,795,595. These processes, however, which represent the state of the art, have serious difficulties to some extent.
For instance, the wet chemical processes involve several operating steps which require the use of different chemicals, long processing time, and therefore are costly.
Electrolytic processes for producing tin (II) sulfate which operate without membranes are uneconomical because, although tin goes into solution anodically, it is deposited at the cathode at the same time. By reversing the polarity of the electrodes, the previously deposited tin can be brought into solution again. However, the process of direct tin deposition at the respective cathode is not eliminated by such a procedure.
The electrolytic processes for producing tin (II) sulfate with organic membranes are disadvantageous because organic membranes have a small pore volume relative to the external total volume of such membranes. In addition, the pore size is relatively large because of the process used to produce such membranes and because it is not possible to sinter such membranes at high temperatures and thus alter their density. As a consequence, these membranes are permeable to several different ions. Thus, in using membranes of this type, this fact, which is economically disadvantageous due to secondary reactions, must be considered.
Processes for producing tin (II) sulfate by electrolytic means involving the use of organic anion-exchange membranes which are permeable to anions have the disadvantage that essentially only anions pass through the membrane and therefore, the conductivity is determined by the mobility or the equivalent conductivity of such anions. Cations and, therefore, also hydrogen ions or hydrated hydrogen ions which are known to be present in aqueous solutions, cannot pass through such membranes.